1. Field of the Invention
The present invention relates to a liquid ejection head and an image forming apparatus, and more particularly, to a liquid ejection head and an image forming apparatus in which the direction of ejection of liquid can be controlled.
2. Description of the Related Art
As an image forming apparatus, an inkjet recording apparatus (inkjet printer) has been known, which includes an inkjet printer head (liquid ejection head) having an arrangement of a plurality of liquid ejection nozzles and which records an image on a recording medium by ejecting ink (liquid) from the nozzles toward the recording medium while causing the inkjet head and the recording medium to move relatively to each other.
The inkjet head of the inkjet printer of this kind has pressure generating units, each including, for example, a pressure chamber to which ink is supplied from an ink tank through an ink supply channel, a piezoelectric element which is driven by an electrical signal in accordance with image data, a diaphragm which serves as a portion of the pressure chamber and deforms in accordance with the driving of the piezoelectric element, and a nozzle which is connected to the pressure chamber and from which the ink inside the pressure chamber is ejected in the form of a droplet due to the volume of the pressure chamber being reduced by the deformation of the diaphragm. In the inkjet printer, one image is formed on a recording medium, such as a paper, by combining dots formed by the ink droplets ejected from the nozzles of the pressure generating units.
In the inkjet printer, normally, a plurality of nozzles which eject ink directly are aligned in one row, and the ink ejected from a certain nozzle is deposited at a prescribed position. In this case, the depositing position is substantially uniform, and therefore the image resolution of the formed image is dependent on the nozzle pitch. Hence, by narrowing the nozzle pitch in order to form an image of high quality, it is possible to achieve a higher resolution in the image.
As a method for obtaining high-quality images, there has been also another method which increases the number of tonal graduations of the pixels which make up the image. However, in an inkjet system, there are limitations on the number of tonal graduations available for one pixel, and unlike the case of a dye sublimation printer, it is difficult to obtain a high number of graduated tones. More specifically, in order to obtain tonal graduations in one pixel in the inkjet system, it is necessary to adjust the ink ejection volume, and therefore, if it is sought to achieve tonal graduations by means of a single nozzle, the upper limit for the number of tones is generally around 16. In order to increase the number of tones yet further, there have been methods in which inks respectively having dark and light hues of the same color are provided separately, and the number of tonal graduations is increased by controlling the use of these inks. However, even though this special system is adopted, unlike the case of a dye sublimation printer, it has been difficult to obtain 256 tones for each color in one pixel.
Therefore, in the inkjet head, a high-resolution image is generally obtained by increasing the pixel density, as described above. More specifically, there is a correlation between the number of tonal graduations in one pixel and the density of the pixels, and even if the number of tonal graduations is small, provided that the pixel density is high, then it is possible for the image to be perceived as an image of high resolution. Although there are differences among individuals, human visual spatial resolution is normally limited to a resolution of approximately 0.05 mm to 0.1 mm. Therefore, if the image density is 250 dots per inch (dpi) to 500 dpi or greater, then it is not possible to recognize mutually adjacent pixels as separate pixels. Hence, as long as an image has the pixel density of a particular value or above, the method of achieving a high-resolution image may be based on the method of increasing the number of tonal graduations in one pixel, or based on the method of increasing the pixel density, and in the inkjet system, high resolution is normally achieved by means of the latter method. Moreover, even in the case of monochrome printing, it is possible to make the font lines even smoother by increasing the pixel density.
Consequently, at present, a high-resolution inkjet head of approximately 1200 dpi can be developed practically, but if it is sought to obtain an image of even higher resolution, then it is necessary to reduce the nozzle pitch in the inkjet head, as described above. However, since the inkjet head includes pressure chambers and nozzles for ejecting liquid, then there are structural limitations on the extent to which the nozzle pitch can be reduced. Moreover, in order to obtain an image of high resolution at high speed, there is a method which uses an inkjet head having a width corresponding to one edge of the recording medium, such as paper. However, if color printing is to be carried out at an image density of 1200 dpi onto A3 size paper, then approximately 60,000 nozzles are required, and it is extremely difficult to manufacture this inkjet head with good production yield. Further, in order to drive an inkjet head having an extremely large number of nozzles, the control circuit also becomes highly complex, and this causes increased costs and reduced reliability in the inkjet head, and hence, in the image forming apparatus. This problem can be resolved provided that the actions of a plurality of nozzles can be achieved by means of one nozzle.
With regard to the quality of the image formed by the inkjet head, aside from the effects of the number of pixels and tonal graduations described previously, the effects of the quality of each individual pixel formed by the inkjet head are not negligible. More specifically, the ink droplets ejected from the inkjet head deposit on a recording medium to form an image, but the depositing position, shape and size of the deposited droplet that forms a pixel also affect the quality of the image formed. Of these factors, the ink depositing position is particular important since displacement of the depositing position has a large effect on the quality of the image. In the inkjet head, since a plurality of nozzles are normally aligned in one row, then displacement of the depositing position of the ink droplet in a direction that is perpendicular to the direction in which the nozzles are aligned (namely, in the direction of movement of the inkjet head), can be resolved by controlling the ejection timing of the ink. However, any displacement of the depositing position of the ink droplet in the direction which is parallel to the direction in which the nozzles are aligned (namely, in the direction perpendicular to the direction of movement of the inkjet head), cannot be resolved by controlling ejection timing of the ink, and in order to resolve this, it is necessary to control the flight direction of the ink droplet ejected from the nozzles.
Moreover, even in cases where no displacement of the depositing position occurs, by controlling the flight direction of the ink droplet ejected from the nozzles, it is possible to make the ink deposit at desired positions more accurately, and hence the resolution of the formed image can be increased even further.
In view of these circumstances, research has been carried out into controlling the flight direction of ink droplets ejected from nozzles, and one method for achieving this is a method based on electrostatic deflection. In this method, a pair of deflecting electrodes are provided so that charged ink droplets fly therebetween, and the flight direction of the ink droplet is deflected by means of the deflecting electric field. However, in this method, it is necessary to provide deflecting electrodes between the recording medium and the nozzles, and hence it is necessary to provide a large interval between the recording medium and the nozzles. The larger this interval, the greater the external disturbance that affects the ink droplets in flight and the more likely there is to be variation in the flight direction, which results in deterioration in the quality of the image. Moreover, in the method based on electrostatic deflection, since the angle of deflection of the ink flight direction is inversely proportional to the speed of flight of the ink droplet, then the angle of deflection varies depending on the speed of flight of the ink droplet. Consequently, it has been difficult to control the deflection and to thereby obtain an image of high resolution by means of the method based on electrostatic deflection only.
Apart from this method, there are methods for controlling the flight direction of the ink droplets ejected from the nozzles. For example, Japanese Patent Application Publication Nos. 57-185159 and 2005-35271 disclose that a plurality of nozzles that have mutually different ejection directions and eject ink droplets to be unified into one ink droplet are provided, and that by adjusting the speed of flight, and the like, of an ink droplet ejected from each nozzle, it is possible to control the flight direction of the unified ink droplet.
Moreover, ink blockages are liable to occur in the inkjet system since the ink used in an inkjet system is a liquid. Japanese National Publication of International Patent Application No. 2003-505281 discloses an invention which prevents the ink blockages by causing ink to flow inside the pressure chambers.
However, in the inventions disclosed in Japanese Patent Application Publication Nos. 57-185159 and 2005-35271, for one united liquid droplet to be ejected, it is necessary to provide at least two nozzles, two pressure chambers and two piezoelectric elements, and the like, and it is necessary to adopt a composition which ejects at least two liquid droplets. Consequently, it is difficult to manufacture the liquid ejection head of this complicated structure, and moreover it is difficult to achieve this technology in practice since it is necessary to control the nozzles independently and the composition for controlling the flight direction is hence complicated.